The present invention relates to apparatus used for testing cables and, more particularly, to apparatus used for testing cables employed in telephone systems comprising a plurality of twisted-wire conductive pairs exhibiting fixed resistance and capacitance per unit length. The apparatus test for the location of a high resistance short between adjacent conductive wires.
Typically, a telephone cable comprises a bundle of twisted pairs (each pair comprises a "tip" and a "ring"). These cables may be buried in the ground, located in underground conduits, or supported overhead on telephone poles. Two types of conductor insulation is found in telephone cable. Older cables use paper insulation on each individual wire. In newer cables, each individual wire has plastic insulation. One common problem in telephone cables is the existence of a physical short between adjacent wires. In telephone jargon, a "short" occurs between two wires of the same pair, whereas a "cross" is between two wires of different pairs; the term "short" is used in the following description to refer to either of these conditions. A low resistance short can seriously affect or entirely disrupt communication over the pair; a high resistance short causes lesser, but still disturbing communication problems. Either of these shorts is commonly caused by the presence of moisture within the cable. With paper insulated conductors, the moist paper results in a high resistance short. With plastic insulation, there may be manufacturing defects in the insulation, referred to as "pin holes", which in the presence of contaminants and water can cause corrosion and a high resistant fault.
There are several methods for locating shorts. If the short is low resistance, i.e. 100 ohms or less, a fault locating technique known as "exploring" is commonly used. This approach is illustrated in FIG. 1. The cable 10 has a conductive pair 12 having a short 14. A tone generator 16 applies an audio tone to the pair 12 which may be picked up through an inductive sensor 18. The sensor is usually connected to an amplifier and a head set so that the craftsman may listen to the tone being picked up from the pair. At position "A", the sensor 18 picks up a tone signal of a relatively strong intensity. At point "B", however, which is past the short 14 (as measured from the tone generator 16), the tone which is picked up has a lesser intensity since the majority of the current is shunted through the short 14, whereby little current appears at point "B" and a dramatic change in signal level is heard. While this approach works quite satisfactorily for low resistance shorts, a resistance of several thousand ohms, for example, 3,000 ohms in a typical cable, will result in a change in signal volume undetectable to the craftsman.
If the resistance is high, i.e. 3,000 ohms and above, there are, in practice, three methods for attempting to locate the fault. In the first method, the craftsman attempts to product a "hard" fault, i.e., reduce the resistance of the fault. This is done through the use of a "breakdown" test set that applies a 700 volt, 5 amp direct current to the pair having the fault. The high resistance short acts like a resistive heater which can burn away the paper insulation from the conductors and "weld" the two wires together producing a lower resistance short which can then located as above with the exploring method. There are two problems in using the breakdown test set. First, there are safety problems in working with the high voltage source, to the craftsman and, unless all drop wires which are connected to the pair are removed in the section being "burned", the high voltage may also result in damage to a subscriber's premises or to the subscriber personally. Second, the method does not work satisfactorily with plastic insulated conductors because the causing of a breakdown at the fault requires even higher voltage than with paper insulation and it is possible to create a new fault at a second location during the attempted "burning".
A second common technique for locating a high resistance fault is the use of a resistance bridge. Various types of modified Wheatstone bridges have been used in the telephone industry over the years; one type is used for the specific application of fault location. The bridge method involves basic techniques which are well known in the art. There are, however, several problems with the bridge technique. One of the principal problems is that these devices do not precisely locate the fault but are rather approximate indicators of the fault location. Despite increased accuracy of modern bridges, there is still a need for precise location since opening the cable is a laborious and costly undertaking. Another difficulty with the use of a resistance bridge is that it requires the craftsman to go to the far end (a point beyond the fault) to "strap" the faulted wires to form the bridge; this procedure, of course, costs time and money.
The third method for locating high resistance faults utilizes time domain reflectometry; the equipment is often referred to as a cable "radar" test set. This method is also well known in the art but has its unique problems. One is the craftsman expertise that is required to interpret signal displays to identify faults and further to measure the distance to the fault. Furthermore, there is the problem of precision as with the above-described resistance bridge technique.
It is therefore the object of the present invention to provide a method and apparatus for testing cables for the location of a high resistance short which is easy to use and highly accurate.